1. Field of the Invention
The present invention relates to a motor vehicle charging control unit and method for controlling a voltage generated by a generator in accordance with temperature.
2. Description of Related Art
A conventional charging control unit, as illustrated in FIG. 1, includes resistances 60, 61 and 63, a Zener diode 62 and a transistor 64 by which in some cases a negative temperature characteristic is created in a charging voltage generated by generator 1. See U.S. Pat. No. 3,718,849.
In the prior art charging control unit, a charging voltage VI applied to a battery 5 is given by the following formula in which a base current of the transistor 64 is ignored. EQU V.sub.B =V.sub.BE (64).times.[(R.sub.60 /R.sub.63)+(R.sub.60 /R.sub.61)+1]+V.sub.Z (62).times.[(R.sub.60 /R.sub.61)+1] (1)
where V.sub.BE (64) is the base-emitter voltage of transistor 64, V.sub.Z (62) is the breakdown voltage of Zener diode 62, and R.sub.60, R.sub.61 and R.sub.63 are the resistance values of the resistances 60, 61 and 63, respectively.
Next, the temperature characteristic of the charging voltage is obtained by: EQU dV.sub.B /dT=(dV.sub.BE (64)/dT).times.[(R.sub.60 /R.sub.63)+(R.sub.60 /R.sub.61)+1]+(dV.sub.Z (62)/dT).times.[(R.sub.60 /R.sub.61)+1](2)
As is obvious from formulas (1) and (2), the temperature characteristic dV.sub.B /dT of the charging voltage and the level of the charging voltage V.sub.B itself cannot separately be set. The setting of the temperature characteristic and the charging voltage involves a matching process with respect to resistances 60, 61 and 63. This is a very difficult process requiring many steps.